Mulan may not be a fairy-tale character, but neither is Peter Pan (and all other Neverland-people) or the people from Wonderland (Mad Hatter and Red Queen) or Robin Hood. Still, Peter Pan and Alice in Wonderland and Robin Hood are all in Disney movies. ;)

Mulan is an ancient fairy tale from China, just like Sleeping Beauty and Snow white are anicent fairy tales from Europe. They're in the general domain and no one owns them or has to give permission to use them. The only thing Disney owns is their particular version of Snow, Beauty and Mulan. (costume, styling, etc). So if Once Upon a Time Mulan started wearing a green robe and carrying a birdcage with an ant in it, then Disney would have to give blessing.